His research uses large spectroscopic surveys of galaxies to understand their formation and evolution, to map their large-scale structures and motions, and to measure cosmological parameters. Prof. Colless is a Fellow of the Australian Academy of Science, an Honorary Fellow of the Royal Astronomical Society, a Fellow of the American Association for the Advancement of Science, an ISI Citation Laureate, a former Vice-President of the International Astronomical Union, a former Founders’ Chair for the Giant Magellan Telescope project, and currently a member of the European Southern Observatory Council. He was made an Officer of the Order of Australia in 2023 for "for distinguished service to scientific research, particularly to astronomy and astrophysics, and to professional societies".